1. Field of the Invention
The present invention relates to a superconducting rotor for generators or motors, and more particularly to a superconducting rotor with a cooling system whose field coil is in more simplified structure to be superconductive.
2. Description of the Related Art
As well known to those skilled in the art, a rotor comprising a field coil made of a superconducting wire, not a copper wire, is called a superconducting rotor, which is shown in FIG. 1.
As shown in FIG. 1, the conventional superconducting rotor comprises: a superconducting field coil 10 made of a superconducting wire for generating a strong magnetic field; a field coil supporting member 11 for supporting the superconducting field coil 10; a torque tube 12 connected to the field coil supporting member 11 for transferring a rotating force from the superconducting field coil 10 to the outside; an inner rotor cylinder 13 for enclosing at least the field coil supporting member 11 including the superconducting field coil 10; an outer rotor cylinder 14 for enclosing the inner rotor cylinder 13; a thermal radiation shield 15 mounted between the inner rotor cylinder 13 and the outer rotor cylinder 14; and a coolant supplying/receiving unit 20 for supplying a cryogenic coolant to the inner rotor cylinder 13 to cool the field coil 10 so that the field coil 10 is maintained at a superconducting state. Between the inner rotor cylinder 13 and the outer rotor cylinder 14 is defined a high vacuum layer 18 for insulating heat transferred from the outside of the rotor.
The coolant supplying/receiving unit 20 is installed to cool the field coil 10 so that the field coil 10 is maintained at a superconducting state. The coolant supplying/receiving unit 20 comprises: an inner tube (a coolant supplying tube) 21, connected to the inner rotor cylinder 13, for supplying the coolant to the interior of the inner rotor cylinder 13; and an outer tube (a gas discharging tube) 22, concentrically disposed around the inner tube 21, for discharging the evaporated gas from the inner rotor cylinder 13.
In the coolant supplying/receiving unit 20 is disposed a rotary sealing unit 30, for example, a ferro-fluid magnetic sealing device, which is operated at cryogenic temperature, for maintaining high vacuum state between the rotatable part of the rotor including the inner and outer rotor cylinders 13, 14 and the high vaccum layer 18 and the non-rotatable part, namely, a vaccum layer 28 of the coolant supplying/receiving unit 20.
A cooling mechanism for maintaining the field coil 10 at a superconducting state in the conventional superconducting rotor shown in FIG. 1 is operated as follows. A coolant 23 at cryogenic temperature stored in a gas buffer tank 25, for example, a coolant having a boiling point of 20 to 30 Kelvin, at which the field coil made of a high-temperature superconducting wire is cooled, such as liquid neon of 27 Kelvin or −246.15 , liquid hydrogen of 20 Kelvin or −253.15 , etc., is supplied to the inner rotor cylinder 13 via a coolant supplying tube 26, the rotary sealing unit 30, and the inner tube 21. The liquid coolant at the cryogenic temperature cools the superconducting field coil 10, which is thus maintained at the superconducting state. The coolant, which is evaporated as the field coil 10 is cooled, is received to a cooling zone 24 via the outer tube 22, the rotary sealing unit 30, and a coolant receiving tube 27, and then condensed at the cooling zone 24. The condensed coolant at the cryogenic temperature returns to the gas buffer tank 25, which will be used again as a coolant for the superconducting field coil.
The evaporated coolant, which is discharged as mentioned above, cools the inner rotor tube 13, the thermal radiation shield plate 15, and the torque tube 12.
The operation of the conventional superconducting rotor shown in FIG. 1 will now be briefly described. First, a coolant at cryogenic temperature is continuously supplied to the inner rotor cylinder 13 via the inner tube 21 to maintain the field coil 10 at a superconducting state. The resulting evaporated coolant is discharged via the outer tube 22. At the same time, the field coil 10 is excited by means of an external power source so that a strong magnetic field is generated. By means of the rotary magnetic field, an electric current is generated at an armature wound around the outer rotor cylinder while being spaced apart from the outer rotor cylinder (for a generator). Otherwise, an electric current may be applied to the armature so that the field coil to generate torque (for a motor).
A superconducting generator or a superconducting motor using the aforesaid conventional superconducting rotor has advantages in that its loss is reduced by more than 50% as compared with conventional normal conducting generators and motors, and that its capacity is double those of the conventional normal conducting generators and motors, assuming their sizes are the same, or its size is decreased by half compared to the conventional normal conducting generators and motors, assuming their capacities are the same, since a large amount of electric current can go through the field coil, whereby it is possible to realize a superconducting generator or a superconducting motor with large capacity and/or small-sized structure. Recently, performance of an oxide-based superconducting wire having high operating temperature has been surprisingly improved, and thus it is predicted that generators and industrial motors having small or medium capacity will be developed recently.
However, the conventional superconducting rotor as shown in FIG. 1 requires the coolant supplying/receiving unit 20 having a complicated multiple cylindrical structure to maintain the field coil 10 at a superconducting state, and a long cryogenic temperature flow channel is formed in the course of supplying/receiving/recondensing of the coolant, whereby a thermal insulating structure is excessively extended, and thus its cooling efficiency is abruptly reduced. Furthermore, the rotary sealing unit is required to maintain vacuum states between the rotatable part of the rotor which is maintained at the cryogenic temperature and the non-rotatable part of the rotor. In this case, the amounts of thermal contraction of the rotating and non-rotating parts of the coolant supplying/receiving unit 20 are different from each other due to a temperature difference between them, which may cause the rotor to be vibrated when it is rotated. Moreover, joints of the rotatable and non-rotatable parts of the rotor may be not reliable when the rotor is operated for a long time while being maintained in the high vacuum state and at the cryogenic temperature.